1. Technical Field
The present disclosure relates to a robotic gripper for use in capturing satellites in zero-gravity and microgravity space environment by gripping the launch adapter interface (alternatively known as “payload launch adapter,” “separation ring,” “payload attach fitting,” “Marman ring,” etc.) which is part of the satellite. The particular structure of the gripper is optimized for efficiently and safely engaging with the Marman ring, in a variety of configurations, for different satellites.
2. Introduction
In recent years, a growing interest in in-orbit servicing of space assets has led to a number of studies and demonstrations, both terrestrial and orbital, to prove the feasibility of sending a servicing spacecraft to rendezvous with, capture, service, and release a client asset, Such servicing may include refueling of propellant, replenishing pressurant gases, repairing or replacing solar arrays or batteries, changing instruments or other payload elements, installing auxiliary sensor or attitude control packages, etc. As more demonstrations are successfully completed, and the cost-benefit tradeoff between servicing an already-orbiting asset versus launching a replacement vehicle and disposing of its pre-existing counterpart demonstrates the economic feasibility of servicing, it has become apparent that an international market for an in-orbit servicing infrastructure is rapidly emerging. Among all subsets of activity under the in-orbit servicing umbrella, there is a common need for the servicing spacecraft to acquire and dock with the client vehicle. In order to interface with a satellite or other space asset, the servicing vehicle must interface with the client in a way that will not damage the client or cause it to be significantly disturbed from its stable orbit attitude. Currently, two schools of thought have dominated the discussion on how best to establish a physical connection between servicer and client; the first being direct docking (berthing) between vehicles, whereby in the most commonly seen proposals the servicer executes a mechanical coupling to either the client's Marman ring (if available) or the nozzle of the client's liquid apogee engine/motor (LAE/LAM); the second being robotic capture, whereby one or more dexterous robotic arms, equipped with a gripping device, reach out and engage any of the aforementioned client interfaces, then pull the client into a more substantial berthing structure.
This disclosure relates to the latter method of rendezvous, and specifically to structures for a gripper that can initiate capture of a client satellite through grasping its Marman ring. A typical Marman ring consists of an aluminum annulus, commonly ranging from 937 to 1,666 mm ire outer diameter, which is structurally integrated to the space vehicle, often to the central propulsion module of the spacecraft bus structure. The outer diameter of the ring features an angled flange, commonly 15° offset from the flat mating surface of the ring. The mating launch vehicle payload attach fatting features a similar ring also possessing an angled flange, such that when the satellite is mated to the launch vehicle, the angled flanges form a “V” in cross-section whose apex lies on the separation plane between the two rings. A clamp band (alternatively known as a “V-band” or “Marman band”) features an array of shoes, mounted in a radially symmetric pattern along separable straps; the shoes feature an inverted “V” cross-section such that when the straps are positioned around the mated rings and fastened together, the shoes engage the angled flanges of both satellite and launch vehicle rings, clamping the rings together in rigid, preloaded contact. The straps are fastened together with one or more bolts or studs that may be cut or released using a pyrotechnic bolt cutter, non-explosive actuator, separation nut, or other suitable means for quickly releasing the strap preload, causing the shoes to quickly spread radially outboard from the rings and thereby freeing the satellite to be released form the launch vehicle at the end of the ascent phase. The mating surface of the ring may feature an annular track positioned coaxially with the outer diameter which receives a corresponding annular protrusion from the launch vehicle in order to react to shear loads during launch and ascent, or the track/boss pair may be furnished in the reverse. The mating surface is often furnished with a chemical conversion coating for electrical bonding, whereas the angled flange may be anodized or otherwise coated to reduce friction and resist cold-welding with the Marman band shoes. The ring may also feature keyways, various pressure pads for receiving the plunger of one or more kick-off devices, and/or umbilical connectors for supplying power and data communication between launch vehicle and satellite. Those skilled in the art will acknowledge other similar methods of attachment between satellite and launch vehicle, such as burn-wire separation rings, etc.
The Marman ring was chosen as the target grasping interface due to a) its nearly ubiquitous use on both government and commercial satellites; b) relative accessibility due to the fact that the surrounding area must be clear of obstructions for satisfactory interface to the launch vehicle; and c) the standardization of ring geometry across multiple launch vehicle platforms.
Next are discussed known structures for grippers. Most commercially available industrial/laboratory grippers do not execute a grasp; they perform a pinch, wherein grasping implies that three rotations and three translations are all constrained by discrete motion stops, and pinching constrains one translation and to some extent two rotations, with all other translations and rotations constrained solely by friction.
Pinch-type robotic graspers are commonly manifested in the form of a parallel-jaw pincher or a rotary-jaw claw. Occasionally, rotary jaws will be outfitted with tips that are suspended on four-bar linkages in such a way as to impart substantially parallel motion through the entire jaw stroke, but the tips still undergo a net arcing motion relative to the grasper chassis, meaning that objects of different widths are pinched at different heights relative to the chassis. Large clamp preloads are employed in order to produce enough friction to constrain all degrees of freedom—this tends to result in over-designed mechanisms that are not optimized for mass or volume.
Known are numerous descriptions of underactuated mechanisms whereby a more conformable grasp permits a mechanism to interface to objects of varying shape and size.
Industrial robotic grippers are commonly actuated via hydraulic and pneumatic systems, both of which are inappropriate for space applications. Motor-driven grippers often employ brushed DC motors, which are not preferred for use in the vacuum of space due to the thermal and contamination concerns with carbon brushes. Also, owing to the ability to provide constant lubrication, motor-driven grippers often feature leadscrews (as opposed to ball screws) as the power transmission element; in space, a leadscrew comprises a single-point failure; being a device that relies on sliding surface action with a finite lubricant supply, leadscrews decrease system reliability, especially in a fully-reversing, high-cycle element such as a gripper. Additionally, industrial grippers often feature oil-impregnated bronze bushings and T-slot or dovetail linear guides for the parallel jaws, all of which reduce system reliability by introducing sources for jamming and galling. Compatibility among coefficients of thermal expansion between different materials is usually not a consideration, and therefore clearances between moving parts, as well as magnitude of interference fits, are not afforded the same rigor of attention as in space flight designs.